It's because you're fighting a team on Auto with the ability to retreat for the "perfect run." It's quite easy to defeat someone who has more power than you by simply observing what the AI will do on Auto, then retreating, and interrupting the mysteries in the next run. You also have 3 teams to do so, and they recover a bit of HP after every battle.
Survival Trial is the only thing that I know, where you can get a FULL ninja or the equivalent in fragments towards that 5-star. It also can give a Seal Scroll on the 12th clear, and other goodies through breakthrough. Considering those rewards, making it SLIGHTLY more difficult is a minor thing.